Recording and reproducing device



June 19, 1945. 5. J. BEGUN 2,378,388

RECORDING AND REPRQDUCING DEVICE Filed Jan. 1, 1942 4 Sheets-Sheet l 6 MfquAuzzR MODULATOR AMPLIFIER FILTER AMPLIHER BP'HLTER June 19, 1945.

5. J. B EGUN RECORDING AND REPRODUCING- DEVICE Filed Jan. 1, 1942 4Sheets-Sheet 2 88w Ra June- 1 9, 1945. s. BEGUN 2,378,333v

RECORDING AND REPRODUCING' DEVI CE" Filed Jan; 1, 1942 4 Sheets-Sheet 3ffi 3 I ZTTORNEY June 19, 1945. 5. J. BEGUN 2,378,388 I RECORDING ANDREPRODUCINGI DEVICE Filed Jan. 1, 1942 4 Sheets-Sheet 4 ATTORNEY.

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Patented June 19, 1945 RECORDING AND REPRODUCING DEVICE Semi JosephBegun, Cleveland Heights, Ohio, as-

slgnor to The Brush Development Company, Cleveland, Ohio, a corporation01' Ohio Application January 1, 1942, Serial No. 425,304

31 Claims.

This invention relates to systems and methods for studying transientphenomena and more particularly to such systems and methods forrecording on a recording medium a transient circuit condition andutilizing the record of the transient for studying the characteristicsof the transient. v

Among the objects of the invention are novel systems and methods of theforegoing type which operate to make a continuous obliterable record ofthe circuit conditions under study on an endlessly-operatingcyclically-effective recording medium, such as a magnetic recordingmedium, so that the circuit conditions are continuously recorded; thecontinuous recording operation being accompanied by a continuousobliteration of the recordings and the obliterating action being stoppedupon the occurrence of a transient condition so as to retain on therecording medium a record of a signal corresponding to the transientoccurrence and the recording operation being discontinued after thetransient occurrence has been recorded on the medium in such manner asto enable the study of the transient occurrence by cyclicallyreproducing the record and utilizing the reproduced record for operatinga cyclically-operative indicating device, such as an oscilloscope, whichpresents an image corresponding to the reproduced signal upon each cycleof the recording medium; such systems and methods which are effectivefor studying low frequency and direct current transients by modulatingwith the signal which is to be recorded a source of carrier oscillationsof a frequency which is reproducibly recordable on the endless magneticrecording medium, so as to provide a modulated carrier signal which iscontinuously recorded on the endless recording medium and obliteratedimmediately thereafter, the obliterating action being stopped upon theoccurrence of a transientthe reproduced recorded modulated signal beingmodulated so as to cyclically provide a demodulated reproduced signalcorresponding to the transient signal which is utilized for operatingthe cyclically operative indicating device; and various other novel anddesirable features forming part of such systems.

The foregoing and other objects of the invention will be best understoodfrom the following description of exemplifications thereof, referencebeing had to the accompanying drawings where- Figure 1 is anillustration of my invention in block diagram form.

Figure 2 is an illustration of another form of my invention, also inblock diagram form.

Figure 3 is a diagrammatic illustration of a modulated carrier currentand a magnetization curve.

Figure 4 is an illustration of the circuit of Figure 2, showing thetransient recording circuit in detail and the trigger circuit and thetransient reproduction circuits in block diagra form and connected tothe transient recordin cuits in block diagram form.

Figure 6 is an illustration of the circuit used for reproducing thetransient, and showing the recording and trigger circuits in blockdiagram form and connected to the reproducing circuit.

Figure '7 is a plan view of a magnetic head in its open position.

Figure 8 is a plan view of the magnetic head in its closed position.

Figure 9 is a diagrammatic illustration of a timing device utilizing mytransient recording device.

My invention comprises means and a method for recording a transientsignal'on a storage medium and repeatedly reproducing it therefrom atvregular intervals to establish a wave, each cycle of which issubstantially a facsimile of the original transient. The device may useany recording medium, and suitable means for recording the transientthereon and reproducing the transient therefrom. I prefer to use amedium which can be used over and over again such, for example, as amagnetic tape or disc, etc.

Such preferred embodiment is illustrated in Figure 1, in which is shownmeans for a method of storing a transient by electrically recording iton an endless magnetic tape l0, means for and a method of reproducingthe recorded transient from the recording medium and presenting itvisually by repeatedly reproducing it on an oscillograph I I or otherindicating device; and associated circuits for controlling theoperations of the 40 means and method. Electrical recording may ineludeelectromagnetic, photoelectric, or any other process or device utilizingelectricity in the recording and/or reproducing process.

The'endless tape I0 is driven about rollers 12 and 13 by a motor l4. Amagnetic obliterating head I5 and a magnetic recording head l6 areassociated with the tape Ill. The obliterating head I5 is adapted toapply a steady saturating flux (such as the flux derived from battery H)to the tape ID as the tape passes, and to bring each incremental portionof the tape to a state of substantially uniform saturation. With theswitch 23 closed in its downward position the recording head It isconnected to a signal recording circuit which may include a source I 8of polarizing current. The transient signal to be recorded on the tapeI0 is applied at the input terminals I9, 20, of the recording circuit.Included in the recording circuit is an amplifier 2| and an equalizer22. The equalizer 22 is to provide substantially constant current to therecording head IS. The instantaneous current which is in the recordinghead I6 is given by the superposition of the instantaneous values of thetransient signal current and the polarizing current. As the saturatedtape passes the recording head a magnetic pattern corresponding to thecurrent in the recording head is recorded on the tape in the well-knownmanner, and as soon as the transient has been recorded the obliteratingand recording process are stopped by automatically or manuallyopening'switches 23 and 28 so that the recorded transient will not beerased or have further transients superimposed upon it. A playback headseparate from the recording head l6 may be used in association with theendless tape III by connecting it to an equalizer 24, an amplifier 25,and the oscilloscope Ii vide the switch 23 for switching the head I6from the recording circuit to the playback circuit.

After the transient, which may occur only once,

-is recorded on the endless tape ill, the tape can be driven around therollers l2, l3 and, with switch 23 thrown to cause the head IE to act asa playback head, each complete transit of the tape will cause the signalrecorded on the tape to produce a trace of the transient on theoscillograph I I. In this manner repeated reproduction of the transientsignal on an oscilloscope II, in which the horizontal sweep of theoscilloscope is synchronized with a point on the tape ill, will producea sufiiciently bright and sustained trace to enable a person to studythe transient, and no time need be lost between the recording andplayback operations other than the time necessary to throw a switch.

If the transient is of an acoustical nature it may be converted by meansof an electromechan- With the above described method it is diflicult toreproduce the recorded signal from the magnetic medium if the signal haslow frequency components as the rate of change of flux in the magnetichead is low for the low frequency components. For the above describedmeans and method of recording this makes necessary the use ofconsiderable equalization for low frequencies with consequentintroduction of phase shift and background noise. With this method thequick interruption of the recording and obliterating current by throwingswitches 23 and 28 after the signal transient is on the tape causes anundesired transient to be recorded on the tape which is apt to interferewith the desired signal transient.

In order to obviate these undesirable conditions I use a signalmodulated carrier current to record on the tape. ihis method permits therecording without phase distortion of frequencies substantially down tozero frequency corresponding to direct current, and can be arranged toavoid introduction of undesirable transients caused by switchingoperations. Several methods of modulating a carrier current to record onthe magnetic medium may be used; among them being amplitude modulation,frequency modulation and phase modulation. A detailed description of onecarrier method of recording may be found in my application entitledApparatus and method for magnetic recording, Serial No. 399,909, filedJune 26, 1941.

Figure 2 illustrates a form of my invention adapted to record transientsin direct current circuits or circuits normally carrying no current, andusing the method detailed in the above-clescribed patent application.Examples of transients which this particular form of the apparatus isadapted to record are the current in spot welders and the charge ordischarge of condensers, etc.

The input terminals I9, 20 are connected to the circuit in which atransient is expected, such for example, as across a low resistance inthe circuit of an impact welder. The motor il drives the roller l2 tocause the endless magnetic tape III to continuously rotate about rollersl2 and I! at a uniform velocity in the direction shown by the arrow. Thetime of one cycle of the tape loop preferably is slightly longer thanthe transient which may, for example, be of about one-tenth of a secondduration, thereby establishing on the tape loop a record of the normalcircuit conditions and a record of the transient condition. Anoscillator 26 is tuned to a frequency somewhat higher than the highestfrequency component to be recorded. When the highest frequency componentof the signal is in the neighborhood of 500 cycles per second a carrierfrequency on the order of 2000 cycles per second is satisfactory. Theoscillator is connected to and caused to excite an obliteratingamplifier 21, which in turn supplies 2000 cycle current to theobliterating head l5. As the tape l0 passes the obliterating head l5each incremental portion of the tape is subjected to a 2000 cyclealternating magnetic force which is sufficiently strong to saturate thepart of the tape closest to the coils. As each incremental portion ofthe tape leaves the coils the magnetic field active on it is graduallyreduced and thus the successive increments of the tape are brought to anunmagnetized state. In this condition the tape is ready to have a newrecord iliipressed on it at the magnetic recording head The oscillator26 also supples 2000 cycle carrier current to the magnetic head IE, butbefore reachingthe magnetic head I6 the carrier current passes through amodulator 29 to which is also connected the transient signal inputterminals I9. 20. In the modulator 29 the transient signal modulates the2000 cycle carrier current from the oscillator, and the signal modulatedcarrier then passes to an amplifier 30. The modulator 29 is of thebalanced type arranged to balance out the signal and pass the modulatedcarrier. A high pass filter 3I- may be connected in the recordingcircuit between the amplifier 30 and the magnetic head iii. Forrecording transients whose highest frequency components are in theneighborhood of 500 cycles per second the high pass filter may bedesigned to cut off somewhat below 1500 cycles. The exact point at whichthe high pass filter cuts on. depends upon the frequency range of thetransient to be recorded and upon the chosen carrier current frequency.The filter 3i is not essential but is useful in taking out hum and theresidual transient signal which may have gotten through the modulator29. If no transient is being recorded the 2000 cycle alternating fluxset up in the magnetic head I6 by the carrier current causes a 2000cycle magnetic wave to be recorded on the moving tape Ill. Upon nearlycompleting a. circuit of rollers I2, I 3, the recorded wave is removedby the obliterating head l5.

When a transient signal is applied to the signal input terminals I9, 20,the carrier current is modulated by the transient and a correspondingmodulated magnetic carrier wave is recorded on the tape. To preventobliteration of the transientmodulated magnetic carrier wave, and toavoid superimposing the carrier on the recorded wave after the tape H]has completed a cycle around the rollers l2, l3, a transientpperatedcontrol circuit is energized to block the obliterating amplifier 21 andto open the recording circuit after suitable time intervals.

The gain of the amplifier 30 which feeds the magnetic head I6 is presetto swing the recording flux set up in recording head l6 by the carriercurrent, over the proper part of the magnetic curve of the tape. (Thisis explained in detail in connection with Figure 3.) The strength of thetransient signal input may be adjusted by a potentiometer 32 connectedacross the signal input terminals 19, 20.

The transient operated control circuit consists of a trigger device 33connected to the transient signal input terminals I9, 20, theobliterating amplifier 21, switching means 34, and sometimes, ifdesired, to a switch 41 connected to the tape drive motor I4. Thetrigger means is actuated by the incoming transient to cause theobliterating amplifier 21 to be blocked thereby preventing theobliterating head I from erasing the record of the transient which ismade on the moving tape I 0, and the actuation of the trigger means 33also causes the switch means 34 to be thrown to break the recordingcircuit to the magnetic head I6 and to make a playback circuit from themagnetic head Hi to the indicating instrument II. If desired the triggercircuit may loe used to break the circuit to the motor l4 and stop thetape l0 immediately after a transient has occurred. Thus, shortly aftera transient has been recorded on the tape I0, the entire deviceautomatically becomes inoperative due to the action of the triggermeans.

For applications in which it is not desired to have an automatic controlof the trigger circuit, an external control 4| is provided. The externalcontrol may be a push button or other similar device for initiating theaction of the trigger device 33 to effect the same results as theactuation of the trigger device 33 by the incoming transient signal.

After the transient is recorded on the tape II] in the form of amodulated carrier magnetic recording, it is ready to be observed byrepeatedly reproducing the tape record on an oscillograph II or otherindicating instrument. From the magnetic pickup head I B the reproducedsignal goes through an amplifier I30 then through a band pass filter 36which is designed to pass, in addition to the carrier, the side bandscorresponding to modulation frequencies up to about 500 cycles. Thesignal then passes to the demodulator 31 where a rectifier detects theoutput of the band pass filter and the carrier and its harmonics arefiltered out, and then to an indicating instrument such as anoscillograph I I.

The output from the demodulator consists of a direct current componentupon which is superimposed a wave composed of a series of transientseach one being substantially a facsimile of the original transient, andfollowing at regular time intervals such that it can be viewed on acathode ray oscillograph.

The oscillograph H shown may be a standard commercial cathode-rayoscillograph, such as type TMV122-B sold by the Radio Corporation ofAmerica, as described in its instruction book IB-23,339, published in1937. As is well known, such standard cathode-ray oscillographs areequipped with a synchronizing switch which is used to control thesynchronization of the horizontal time-axis sweep with the investigatedsignal. The synchronizing switch usually has at least two positions,namely, an internal synchronizing position, in which the time-axis sweepis automatically synchronized with the investigated input signal, and anexternal synchronizing position, in which the time-axis sweep issynchronized with an external synchronizing signal.

The number of repetitions per second at which the recorded transient isrepeated on the oscillograph II depends on the number of tape cycles persecond. For example, this frequency of repetition may be ten transientfacsimiles per second on the oscillograph. The time-axis sweep of thecathode-ray may be internally synchronized with the reproduced,demodulated and illtered signal, which is impressed upon theoscillograph II, or the tape joint 49 or a nick in the tape maybe usedas a trigger for synchronizing the time-axis sweep of the cathode-ray.

In order to record on the steel tape l0 an elec-' tric current is usedto set up a proportional magnetizing force which is applied to the tapel0. Therefore, as the electric current varies in accordance with theinstantaneous value of the signal to be recorded, the magnetizing forceimpressed on the tape varies in proportion to the original signal.

The relationship between the magnetizing force H and the resultantmagnetic induction B for increasing and decreasing values of H isexpressed graphically by a hysteresis loop, and for increasing values ofH starting with a demagnetized tape it is expressed by a virginmagnetization curve. Every magnetic material has a particular virginmagnetization curve and hysteresis loop which identifies the magneticcharacteristic of the material.

From hysteresis loops it is obvious that magnetic effects are notreversible, and that the magnetic induction obtained from a givenmagnetizing force depends upon the previous magnetic history of thematerial on which the magnetizing force is impressed. Accordingly, witha tape upon which previous magnetic records have been made it isnecessary to produce a substantially uniform condition either ofsaturation or demagnetization in order to "erase the magnetic record.

In a. magnetic tape it is necessary to obliterate the previous recordbefore a new record is put on the tape or mixed records are apt toresult. This obliteration may be obtained by applying a magnetizingforce to the tape which is suificiently high in value to cause aninduction which is in the saturation range on the magnetization loop ofthe material. That is, in the neighborhood of point S1 or S2 in Figure3, in which the heavy line 38 represents a typical virgin magnetizationcurve for a material which has been demagnetized. The obliteration mayalso be obtained by applying over a relatively large area" of the movingtape an alternating magnetizing force derived from an alternatingcurrent, and gradually reducing the magnetizing force to zero to reducethe residual magnetism to substantially zero.

Subsequent magnetization in accordance with a signal will have a 3-Hrelationship in accordance with the virgin magnetization curve 33 of thematerial. The decrease in strength of the magnetizing force is obtainedfor each portion of the tape by removing that portion of the tapefarther and farther from the center of the magnetizing force. This isthe method of obliteration employed in the preferred form of myapparatus.

Continuous alternating current is applied to the coils of obliteratingpole pieces which are positioned near the moving tape. This causes arapidly reversing diffuse magnetic flux to be impressedthrough an areaof the tape which is large in comparison to the area through which therecording flux is impressed, and as each por tion of the tape moves awayfrom the pole pieces which are the center of the flux distribution, thestrength of the field on each such portion of the tape decreasesgradually to zero. The pole pieces of the obliterating head have theirlike magnetic poles toward each other. However, the use of alternatingcurrent in the coils causes the magnetic polarity of each pole toalternate; at one instant the north poles are toward each other, thenext instant the south poles aretoward each other. This establishes amagnetic field about each pole piece which bucks the other magneticfield and results in a relatively large leakage or diffuse magneticfield through which the tape moves during the obliterating process- Eachportion of the tape which has been removed sufficiently far from theobliterating poles is now reduced to substantially zero residualmagnetism and is ready to have a magnetizing force corresponding to asignal impressed on it. This is done by the recording head indicatedgenerally by the reference character It and comprising two pole pieces,one of which is surrounded by a coil. In the coil there is a currentcorresponding to the signal to be recorded on the tape. The currentestablishes a varying magnetizing force in the pole piece which in turnestablishes varying magnetic flux density in successive portions of thetape.

Figure 3 diagrammatically illustrates my preferred method of recording alow frequency transient on a magnetic materlalwhich has previously beendemagnetized to substantially its virgin state, and comprises modulatinga carrier frequency current 39 by the transient signal to establish twoenvelopes 40, and applying to the tape l magnetizing forces havingsuccessive peak values corresponding to both of the envelopes 40, and byadjusting the modulation of the carrier current by the signal to causeit to be at all times below 100 per cent.

Recording a signal on a tape introduces considerable distortion due tonon-linearity of portions of the magnetization curve of the tapematerial unless steps are taken to cause the recording to be only on thesubstantially linear portions of the magnetization curve. I effectrecording on the substantially linear portions of the magnetizationcurve of the tape material by applying to the coils a current comprisedof a carrier current modulated by a signal to be recorded, the highestvalue of the modulated carrier current being insufficient to causemagnetic saturation in the tape l0, and the percentage of modulation ofthe carrier by the signal being at all times substantially less than 100per cent.

I have found that for most carrier frequencies the same oscillator canbe used as a source of both carrier and obliterating current. Thiseliminates the need for an extra oscillator and makes a more simpledevice. However, if the need arises it is to be understood that theobliterating current and the carrier current may be produced by separatesources (such as oscillators) and that the obliterating currentfrequency may be of any value which will give a suflicient number ofreversals of polarity to reduce the residual magnetism in' the tape tosubstantially zero.

In Figure 3 the line 38 represents a magnetization curve for a materialwhich may be used as a magnetic tape. The curve comprises twosubstantially straight portions 232 and 233 and three curved portions234, 235, and 238. The curved portion 236 lies between the twosubstantially straight portions 232 and 233 and represents nonlinearitybetween the magnetizing force and the induced fiux density for smallvalues of the magnetizing force. From zone 231, which is substantiallythe junction of curved line 236 with line 232, to zone 238, which issubstantially the junction of line 232 with curved line 234, the line232 is substantially straight. This means that for each unit increase inmagnetizing force H between zones 231 and 238' there is substantially aproportional increase in the flux density B in the tape I0 from 231" to238". Above zone 238 partial saturation takes place and B does notincrease on a substantially constant ratio with H. Below the H axis thesame effects take Place for magnetizing forces created by currents ofthe opposite polarity. Zone 239 represents the junction of lines 236 and233; and zone 240 represents the junction between lines 233 and 235.Zones 231, 238, and 239, 240 may be referred to as critical zones orpoints as they define substantially the extremities of the substantiallystraight portions of the magnetization curve.

In order that a signal recorded on the tape Hi can be reproduced withoutconsiderable distortion it is necessary that the maximum magnetic fluxinduced in the tape during each cycle of the modulated carrier currentcorrespond to a point which lies on the substantially straight portion232 or 233 of the magnetization curve of the material.

Figure 3 further illustrates how the magnetizing forces H correspondingto a modulated carrier current induce maximum magnetic flux densities inthe tape l0 which correspond to points on the straight portions 232 and233 of the magnetization curve of the tape material, and how distortionin the reproduced signal may be reduced by preventing the maximum fluxdensities for each cycle in the tape from corresponding to points on thecurved portions 234, 235, and 233 of the magnetization curve of thematerial. This may be referred to as recording on the substantiallystraight portions of the curve. In recording a substantially symmetricaltransient on a substantially demagnetized tape by my modulated carriercurrent system, the greatest dynamic range can be obtained by adjustingthe normal peak amplitude of the unmodulated carrier current to have avalue which corresponds to a magnetizing force H having a value whichlies substantially midway between the value of the forces correspondingto zones or points 231' and 238'. The value of this force is representedby the point having the reference character 245. Point 248 representsthe value of this force for the opposite polarity of the carriercurrent. The minimum amount of distortion due to recording on the curvedportion of the magnetization curve is obtained by adjusting thepercentage of modulation of the carrier current to a value which is atall times sufllciently below 100%: that is, with the normal peakamplitude of the carrier current adjusted substantially midway betweenzones or points 231' and 238 for one polarity and between zones orpoints 239 and 240 for the other polarity, the peak value of themagnetizing force produced by each cycle of the modulated carriercurrent will not be greater than the value of the magnetization force Hrepresented by the points 238 and 240, nor less than the value of themagnetization force H represented by the points 231 and 233. I havefound that 50 to 60 per cent modulation is satisfactory for somemagnetic materials. By this process the maximum values of the fluxdensities, in the tape for each carrier cycle correspond to points whichlie only on the substantially straight portions of the magnetizationcurve of the material, and therefore distortion of the signal reproducedfrom the envelope of the carrier is reduced to a minimum. The manetizing force H corresponding to the amplitude of the peak of eachcycle of the modulated carrier current is always less than the value ofzones or points 236', 240, to prevent recording on the curved portions234, 235 of the magnetization curve, and is always greater than thevalue of zones or points 231', 239' to prevent recording on the curvedportion 236 of the magnetization curve. Depending upon the material usedfor the endless tape the size of the curved portions 234, 235, and 236varies and the degree of curvature of the substantially straightportions varies. For each kind of tape it is possible to adjust theamplitude of the unmodulated carrier current to cause the normal peakvalue of each cycle to fall substantially half way between points 231'and 238 and between 238' and 240', and to adjust the amount orpercentage of modulation of the carrier current by the signal to berecorded to cause the peak value of each cycle of the modulated carriercurrent to lie between points 231' and 238 for one polarity of thecarrier current, and between points 239' and 240' for the other polarityof the carrier current.

In certain applications, the transients to be recorded will all be ofthe same general nature and may consist of unidirectional pulses. Atypical example is the transient voltage developed across a resistorwhen a condenser is discharged therethrough. In such cases, theamplitude of the unmodulated carrier may be so adjusted that theresulting magnetizing forces, corresponding to the peak values of thewave, fall near points 237 and 233 and the transient circuit soconnected that the carrier is modulated upwardly toward but notexceeding points 238 and 240; or downward modulation may be used withunmodulated carrier peaks producing magnetizing forces falling nearpoints 238 and 240. For cases between symmetrical transients andcompletely unsymmetrical transients, intermediate carrier amplitudes maybe used. By following this procedure thus substantially straight portionof the virgin magnetization curve is used to the maximum advantage.

Detailed description of recording circuit Figure 4 is a detailed circuitof a transient signal recording means somewhat similar to the recordingmeans of Figure 2 but differing in that a single amplifier is shown, andrelay 34 does not make the playback circuit.

,Plate power for the various tubes detailed in Fig. 4' is supplied bythe power supply P1 which may be of conventional design.

The modulator 29 comprises a pair of pentode tubes 15, 16 with plate andscreen voltage stabilized by a glow tube voltage regulator 11. Carriersignal supplied by oscillator 26 is applied in opposite phase to thegrids of the two modulator tubes by center tapped transformer 64. Thetransformer secondary 66 is tuned to the oscillator frequency by shuntcondenser 18. The transient signal derived from potentiometer 32connected across input terminals I9 and 20 is supplied in the same phaseto both modulator grids through the connection and the center tap of thetransformer secondary 66. Bias for the modulator tubes is produced bythe plate and screen current flowing through resistor 19. The plates ofthe modulator are connected in push-pull fashion to the center tappedprimary I45 of transformer 85. The secondary of this transformersupplies the modulated carrier to amplifier 30. Since the transientsignal is applied to the two grids in the same phase while the plates ofthe tubes are connected in push-pull fashion, the transient signalitself cannot appear at the secondary of the output transformer,provided that the modulator circult is properly balanced. Presence ofthe transient signal itself in the recording circuit is undesirable asit may interfere with proper recording of the modulated carrier. Ratherthan provide means for accurate balancing, a high pass filter 3| isemployed at the output of the amplifier to filter out any residualtransient signal due to modulator unbalance.

The oscillator 26 employs a dual triode 55. The oscillator circuitproper is of the resistance stabilized Hartley type and employs onetriode section 56 of the tube 55. The frequency determining elements aretapped inductance 59 and condenser 60. Resistor BI is the stabilizingresistor and 63 is a blocking condenser. The plate circuit is suppliedthrough choke coil 42. The tube is biased by means of the cathoderesistor 5| which is bypassed by condenser 52. The second section 51 ofthe tube 55 is employed as a buffer amplifier. Its grid is connectedthrough blocking condenser 46 to the plate end of the oscillator coil 59and has a bias path to ground through grid leak 48. In the plate cathodecircuit are connected three impedances, plate resistance 44, biasresistor 45, and primary 65 of the modulator input transformer 66. Theplate voltage for both sections of the tube is stabilized by the sameregulator tube 11 that functions for the modulator. The amplifiedcarrier voltage developed across resistance 44 is supplied to theobliterating amplifier 21. Carrier current flowing through primary 65supplies carrier signal to the modulator. Resistor 45 supplies bias forthe buffer tube and also introduces some degeneration.

The amplifier 30 is a two stage resistance capacity coupled type ofconventional design. A fixed voltage divider comprising resistors 16 andH is provided at the input to select a suitable portion of the modulatedcarrier output of modulator 29. The output of the second stage amplifiertube 81 is coupled to high pass filter 3| through a suitable matchingtransformer BI and the output of the filter is supplied via switch orrelay 34 to recording head I6. The high pass filter 3| is of a simpleconfiguration, comprising two condensers 88 and inductance 89.

The carrier frequency supplied by the oscillator, and the frequencyresponse characteristics I 05 which is not energized during therecording process.-

A trigger relay circuit (to be described later) is provided to block theamplifier to cut off the obliterating current a short time after thetransient occurs and just before the recording of the transientmodulated carrier reaches the obliterating head. This is done by causingthe trigger circuit to energize relay I05 which breaks the groundconnection for grid resistor I and con- I nects the resistor to anegative bias tap I09 of powe pply P1.

head and is connected to the recording circuit only as long as thetrigger relay switch 34 has not been actuated by trigger means 33 tocause it to become inoperative. Shortly after a transient signal isapplied to the signal input terminals I9, 20 the trigger relay becomesoperative to cause the magnetic head to be disconnected from therecording circuit to prevent superimposing extra signals in the desiredrecorded transient. The actuation of the trigger circuit also blocks theobliterating amplifier which prior to the coming of the transient hasbeen continuously erasing the record of the carrier on the tape I0.Blocking the obliterating amplifier prevents the amplifier from erasingthe transient signal which has just been recorded on the tape.

The use of an obliterating current which gradually reduces the magnetismin the tape III to zero prevents a transient from being recorded on thetape due to the sudden blocking of the obliterating current, and the useof the modulated carrier current recording process prevents a transientfrom being recorded on the tape due to the sudden disconnection of therecording head from the recording circuit.

Obliterating amplifier Figure illustrates the details of theobliterating and trigger circuits in connection with the recording andplayback circuits which are shown in block form.

During the part of the recording process in which the unmodulatedcarrier is recorded on the tape I 0, that is, during the time that theapparatus is in operation in anticipation of the occurrence of atransient, the coils of the obliterating head I5 are energized by analternating current of suflicient strength to saturate magnetically theportions of the tape immediately adjacent the head. For convenience,this obliterating current is obtained from the oscillator 26, a singlestage amplifier 21 being interposed to obtain sufllcient current and toisolate the head from the oscillator. The obliterating amplifier 21comprises a power pentode tube 91 whose plate is coupled to theobliterating head by means of transformer I02. Condenser I04 tunes theobliterating head circuit to the frequency of the oscillator so thatmaximum current may be obtained. Plate current for the tube 91 issupplied by power supply P1 which may be of conventional design. Normaloperating bias is provided by cathode resistor IOI which is bypassed bycondenser III. Oscillator 26 energizes control grid 98 through theconnection 95 and blocking condenser 96. Grid 98 is maintained at theproper negative bias potential by the direct current path to groundcomprising resistor 99 in series with the parallel combination ofresistor I01 and resistor I00. Resistor I00 is grounded through thecontacts of relay stop the obliterating process.

This bias is sumcient to stop the flow of plate current to tube 01 andthereby Condenser I08 is connected into this bias circuit to slow downthe blocking process to prevent the recording or an undesired transientwhich might result if the obliterating process were stopped suddenly.

The obliterating head I5 has two pole pieces, ll6and II6, surrounded,respectively, by coils II! and H8. The two coils are connected in seriesopposing relationship so that at any instant the adjacent ends of thepole pieces have the same polarity, thus causing a difiuse magneticfield in the neighborhood of the tape.

Details of trigger circuit Simultaneously with the recording of thetransient signal on the moving tape I0, the transient actuates a triggercircuit shown in detail in Figure 5 to cause (after a certain timedelay) the obliterating head to stop erasing" the signal which is on thetape, and to cause the magnetic head I6 to stop recording on the tape.The interval between the time when the first part of the transientsignal is put on the tape I0 and the tim when the obliterating andmagnetic heads I5 and I6 cease to be operative should be slightly lessthan the time interval for one cycle of the tape.l0 around the rollersI2 and I3. This assures that there will be no superposition of anothersignal on top of the already recorded transient signal, and if the tapeis long enough the entire transient will be recorded. In other words,the trigger circuit synchronizes the recording cycle with the timeperiod in which the transient occurs; and in addition governs the lengthof time of the recording cycle, this time being a function of the speedand the length of the endless tape. It is essential that overlapping inthe recording process be avoided, meaning that the recording periodshould be stopped before the tape I0 has completed a cycle around therollers I2 and I3. Y

A gas tetrode tube I 33 has in its plate circuit the coils of relay I05which when energized blocks the obliterating amplifier and relay I62which interrupts the recording process. Normal- 1y, this tube isnon-conducting and the relays I 05 and I62 are not energized. Itscontrol grid I58 is connected through variable resistance I6I to thecathode of another gas tetrode I32 which also normally isnon-conducting. Grid I58 is bypassed to ground by condenser I60. Whenthe gas tetrode I32 becomes conducting the voltage drop across cathoderesistor I 51 applies a positive bias to grid I58 thus "firing gastetrode I33 and thereby energizing relays I05 and I62. Resistor I 6| andcondenser I60 introduce a time delay, depending on the values of thesetwo components so that the relays are energized at a predeterminedinterval of time after the firing of tetrode I32. This tube is "fired bythe transient signal and the time delay provided by resistor IN andcondenser I60 is so adjusted that the obliterating and recordingprocesses are stopped just as the tape loop has completed the transientrecording cycle.

It is the nature of gas tetrode I32 to fire and become conducting uponapplication of positive potential to its control grid I56 but not tofire upon application of a negative potential. To insure operation ofthe trigger at the proper time for transients of either polarity, a fullwave rectifier is interposed between the transient input terminals I9,20 and the control grid I56. The transient signal is applied throughblocking condenser I46 to grid I40 of dual triode tube I34 connected asa phase inverter of conventional design. The plates I36 and I43 arecoupled through blocking condensers 20I and 202 to grids I39 and I42 ofthe dual triode I35 which is connected as a full wave biased detector.The plates I31 and MI are connected to power supply P1. The grids I39and I42 are both biased to cut off through the connection of gridresistors I I2, II 3 to a negative bias tap on power supp-1y P1. Due tothe large negative bias no plate cathode current flows in tube I35 sothat there is no voltage drop across cathode resistor I66.

When a transient occurs either grid I39 or I42 becomes less negativecausing plate current to flow in its section of the tube. This currentproduces a voltage drop across resistor I66 which in turn fires triggertube I32. As explained before, as soon as tube I32 fires a voltage isapplied to the time delay network I60, I6I and after a suitable timeinterval the voltage of the grid I58 rises sufiiciently to fire tubeI33, thus energizing relays I and obliterating and recording processes.During the time taken to build up a firing potential at the grid of tubeI33 the transient is being recorded on the tape loop in the form of atransient modulated carrier wave and the operation of relays I05 and I62finally takes place to prevent obliteration of the recorded transientand to prevent superimposing additional carrier signal on the recordedtransient.

In order to reproduce the transient from the moving tape I0, a switchAis thrown from the recording position R to the playback position P. Theswitch A has six blades I2I, I22, I23, I24, I25, and I26 as shown inFigures 4 and 5. In the recording circuit shown in detail in Figure 4,the actuation of the switch A causes blade I2I to break the plate supplyto oscillator 26 which is through the choke coil 42, and causes theblade I22 to break the connection from the recording circuit to themagnetic head I6 and make a connection from the magnetic head I6 to theplayback circuit. The playback circuit may include a Dre-amplifierindicated generally by the reference character I3I, and positionedbetween the magnetic head I6 and the amplifier 30. The amplifier 30 isadapted to amplify the transient modulated carrier signal both before itis recorded on the tape I0 and while it is being played back from thetape. The blade I23 of switch A is adapted to control the input to theamplifier 30. In the recording position the input to amplifier 30 isfrom the modulator 29 and in playback position the input is from thepre-amplifier I3I. The switch blade I24 is located between-the amplifier30 and the high pass. filter 3| and is adapted to direct the output ofthe amplifier 30 to the magnetic head I6 when it is in the recordingposition and to the band-pass filter 36 of the playback circuit when itis in the playback position. The blade I26 is located in the plate I62which stop the supply to the two trigger tubes. In the recordingposition the switch closes the plate circuit so that the tubes may befired by the transient. In order that the tubes may be restored to thenon-conducting condition before making a new recording, the switch I26is arranged to open the trigger tube plate circuits when it is turned tothe playback position. This restores the contacts of relay I05 to theiroriginal condition, removing the blocking bias from the obliteratingamplifier 21. To prevent obliteration when this takes place, a switchblade I is located in the plate circuit of obliterating amplifier tube91 to open this circuit during playback.

Reproducing circuit detail-F g. 6

After the transient signal modulates the carrier current and is recordedon the tape I0, and after the trigger circuit has operated to stopfurther recording and further obliteration, the transient signal, afacsimile of which is recorded on the tape, is played back from the tapeto an oscilloscope II or other indicating device by means of theplayback circuit. Repeated reproduction of the transient on theindicating instrument will give an observer sufficient time to see thetransient even though it originally was a very rapid one.

For the reproduction of the transient, the magnetic head I6, which isthe recording head during the recording process, becomes a playback headdue to switch I22 connecting it to the playback circuit through line205. It is obvious that separate recording and playback heads may beassociated with the tape I0 if it is so desired. Actuation of the switchblade I22 from terminal R to terminal P connects the magnetic head I6 tothe playback circuit which consists of the pre-amplifier I3I, theplayback amplifier I30. (which may be the same amplifier as therecording amplifier the band pass filter 36. and the demodulator and lowpass filter 31, to the recording or indicating instrument II. It isapparent that the single amplifier, which is here indicated by referencecharacter 30 in the recording circuit and I 30 in the reproducingcircuit, may be two separate amplifiers if desired.

The pre-amplifier I3I may employ a single tube I10, and is coupled tothe recording and reproducing amplifier I30 by a blocking con denser HIand a resistor I12. The plate of the tube I10 is connected to powersupply P1 through coupling resistor I69 and filter resistance I68. andthe screen grid I99 is connected to the power supply Pl through resistorI61 and is by-passed to ground by a condenser 209. A filter andisolating condenser 204 by-passes to ground the junction of resistorsI61, I68, I69. The signal input from the magnetic head I6 to thepreamplifier I3I is through line 205 to control grid 206 in the tubeI10. Bias for tube I10 is provided by cathode resistor 208 which isby-passed by condenser 2I0. The tube 86 in the amplifier I30 (Fig. 4)has a grid resistor I13 which, together with the resistor I12,establishes a voltage dividing network to cause the same voltage to besupplied to the amplifier I30 during reproduction as was applied to theamplifier during the recording process. The output of the amplifier I 30is applied through connection I92 to the single T-section band passfilter 36 which is comprised of two series arms each having aninductance I14 and a condenser I15, and a shunt arm having inductanceI16 and a condenser I11.

For a carrier frequency of 2000 cycles per sec- Resistor I98 is providedto maintain grid nd and transient signal frequency components up to 500cycles per second, the pass band of the filter should extend from about1500 cycles per second to about 2500 cycles per second.

The band pass filter 36 is connected to the demodulator 31 by means of atransformer I18, the secondary winding of which is connected to theplates of a double diode rectifier tube I19. The output of the doublediode tube I19 is connected through a resistance I80 to a low passfilter comprised of two M-derived sections I8I and I82. There are twoseries branches and three shunt branches in the filter. The. firstseries branch is comprised of inductor I83 and a condenser I 84, and thesecond series branch is comprised of inductor I85 and a condenser I86.The first shunt branch has a condenser I81, the second shunt branch hasa condenser I88, and the third shunt branch has a condenser I89. thefilter is applied across a resistor I93 and then to output terminals I94and I95, to which may be attached an oscilloscope, oscillograph, orother indicating device lg. One of the sections of the low pass filterhas ts frequency of maximum attenuation at the carrier frequency 2000cycles and the other section has its frequency of maximum attenuation atthe second harmonic of the carrier, 4000 cycles. Adjusting the filtersections I8I and I82 in this manner eflectively suppresses the carrierfrequency and its second harmonic and causes only the transient signalto be applied to the output terminals I94 and I95.

External trigger controL-Fig.

In some applications the use of the internal automatic trigger circuit33 may not be desirable. I, therefore, provide an external triggercircuit which may be actuated by an operator to effect the same resultsas the actuation of the automatic circuit insofar as blocking theobliterating amplifier and stopping the recording process are concerned.Switch I65 is manually controlled and switches the phase inverter out ofthe circuit and connects grid I56 of gas tetrode I32 to the externalcontrol circuit. The external trigger circuit comprises a switch I96which may be operated byhand or by some external circuit. One terminalof the switch I96 is connected to the positive terminal I91 of a lowvoltage D. C. source whose negative terminal is grounded, and the otherterminal of the switch is connected through switch I65 to the controlgrid I56 of the first gas tetrode tube I32. I56 at ground potentialuntil the external control is actuated. Applying positive bias to thecontrol grid I56 causes the tube I32 to fire thereby initiating thesequence of events which blocks the obliterating amplifier 21 and stopsthe magnetic head I6 from recording as has heretofore been described.

Modulation check-Fig. 6

I have provided a switch 200 which connects the oscilloscope II directlyto the output of the amplifier 30 in order to make it possible toobserve-in the oscilloscope the amount of modulation for applicationswhere the transient can be caused at the will of an operator, such forexample as an impact welder. It is possible to regulate the amount ofmodulation in advance of recording a' transient to be studied on therecording medium. If there is too much or too little modulation it canbe adjusted before the The output of the second section I62 of transientsignal is applied. The amplitude of the input transient signal can becontrolled by the potentiometer 32.

Figures '1 and 8 illustrate a type of head I6 which is particularlyuseful for a transient recording device. It comprises a base 2I5 uponwhich is mounted a stationary block M6 and a movable block 2I1. Insidethe stationary block 2I6 there is a coil 2! which is adapted to beelectrically connected into the recording or the reproducing circuits,and inside the coil 2! is a rigid pole piece M9. The movable block 2 isattached to the base 2I5 by means of a screw 220 about which the blockis adapted to pivot to establish an open position shown in Figure 7 anda closed position shown in Figure 8. The magnetic tape I0 is adapted tolie between the movable block 2" and the stationary block 2I8, and whenthe head I6 is in its open position may be slipped out from between thetwo blocks. This construction is particularly valuable for transientrecording devices which may be located in a remote place such forexample as a remote power supply line. The recording device may beconnected to the line and left in an operating condition. If a transientshould occur it will be recorded. For an installation of this type theswitch 41 (Fig. 2) to stop the motor I 4 is valuable. Examination of therecording device at a later date will disclose the presence of therecorded transient and the tape can be removed from the head I6 andtaken back to the laboratory for study. A movable pole piece MI ispositioned within the movable block 2 I 1 and is biased toward the tapeI0 by a spring 222. A spring clip 223 is provided for latching the twoblocks 2| 6 and 2H together when the head I6 is in its operatingcondition.

Figure 9 illustrates diagrammatically a timing device utilizing myinvention and adapted to record the speed of a bullet 225. The timingdevice comprises an oscillator 226 in a bridge circuit having fourresistances 221, 228, 229, and 230. With all four resistances in thecircuit there is no output to the recording head I6 as the bridgecircuit is balanced. The bullet 225, upon being fired, breaks orotherwise alters the resistance 221 thereby unbalancing the bridgecircuit and causing signal output to the recording head I6, which signalis recorded on the tape I0 in accordance with my invention. The bulletthen breaks or otherwise alters the resistance 228 reestablishing abalance in the bridge circuit which stops the recording of the signal.The recorded transient consisting of a few cycles of a known frequencyfrom oscillator 226, may then be reproduced on an oscillograph and thetime of flight from resistance 221 to resistance 228 determined from thenumber of cycles of the signal on the oscillograph and the distance dbetween resistances 221 and 228.

The present application is directed to features of the inventiondisclosed and claimedherein and relating to systems and methods forstudying transient phenomena, and more particularly to such systems andmethods employing an electric circuit for recording on a, recordingmedium a transient circuit condition occurring on the circuit andutilizing the record of the transient for studying the characteristicsof the transient by continuously making an obliterable record of thecircuit conditions under study on an endlesslyoperatingcyclically-effective recording medium, such as a magnetic recordingmedium, so that the circuit conditions are continuously recorded; thecontinuous recording operation being accompanied by a continuousobliteration of the recomings and the obliterating action being stoppedupon the occurrence of a transient condition so as to retain on therecording medium a record of a signal corresponding to the transientoccurrence and the recording operation being discontinued after thetransient occurrence has been recorded on the medium in such manner asto enable the study of the transient occurrence by cyclicallyreproducing the record and utilizing the reproduced record for operatinga cyclicallyoperative indicating device, such as an oscilloscope, whichpresents an image corresponding to the reproduced signal upon each cycleof the recording medium; such systems and methods which are eflectivefor studying low frequency and direct current transients by modulatingwith the signal which is to be recorded a source of carrier oscillationsof a frequency which is reproducibly recordable on the endless magneticrecording medium, so as to provide a modulated carrier signal which iscontinuously recorded on the endless recording medium and obliteratedimmediately thereafter, the obliterating action being stopped upon theoccurrence of a transient-the reproduced recorded modulated signal beingmodulated so as to cyclically provide a demodulated reproduced signalcorresponding to the transient signal which is utilized for operatingthe cyclically operative indicating device; and various other novel anddesirable features forming part of such systems.

The magnetic recording systems and'methods utilizing a signal modulatedcarrier frequency which is reproducibly recordable on a magneticrecording medium, described herein in connection with theexemplifications of the present invention, embody various otherinventive features which are claimed in my copending application SerialNo. 540,667, filed June 16, 1944, as a continuation-in-part of theapplication Serial No. 399,909, filed June 26, 1941.

It will be apparent to those skilled in the art that the novelprinciples of the ,invention disclosed herein in connection withspecific exemplifications thereof will suggest various othermodifications and applications of the same. It is accordingly desiredthat in construing the breadth of the appended claims they shall not belimited to the specific exempliflcations of the invention describedherein.

I claim:

1. In a system for recording a transient circuit condition which mayoccur in an electric circuit at an unpredictable instant and forreproducing the record: an endlessly-operating cyclically-effectiverecording medium; recording means connected to the circuit in which atransient is expected for continuously recording the circuit conditions;means for obliterating the recorded signal; means responsive to anoccurrence of a transient circuit condition for rendering theobliterating means inoperative; means operative a time intervalthereafter for rendering the recording means inoperative so as to retainon said recording medium a record corresponding to said transientcondition; and reproducing means operative to cyclically reproduce therecord recorded on said recording medium.

2. In a system for recording a transient circuit condition which mayoccur in an electric circuit at an unpredictable instant and forreproducing the record: an endless-operating cyclically-effectivemagnetic recording medium; means for continuously supplying signalvoltages of a frequency tions of said circuit; said recording mediumbeing cyclically operated at a predetermined rate correlated to thefrequency of said signal voltages;

recording means for continuously magnetically recording on saidrecording medium a record of said signal voltage and obliterating meansfor returning a portion of said medium to a uniform state ofmagnetization a relatively short time interval after a record has beenmade thereon; means responsive to an occurrence of a transient circuitcondition on said circuit for discontinuing the action Of saidobliterating means on said medium; means operative a time intervalthereafter for rendering the recording means inoperative so as to retainon said recording medium a record corresponding to said transientcondition; and reproducing means operative to cyclically reproduce therecord recorded on said recording medium.

3. In an arrangement for recording a transient circuit condition of anelectric circuit which occurs thereon at an unpredictable instance: anendlessly operating magnetic recording medium; means associated withsaid circuit for continuously supplying signal voltages of a frequencyreproducibly-recordable on said magnetic recording medium andrepresentative of the conditions of said circuit; said recording mediumbeing cyclically operated at a predetermined rate correlated to thefrequency of said signal voltages; recording means for continuouslymagneticallyrecording on said recording medium a record of said signalvoltage and obliterating means for continuously obliterating saidrecord; and means re sponsive to an occurrence of a transient circuitcondition on said circuit for rendering said obliterating meansineffective and for subsequently rendering the recording meansinoperative after a signal corresponding t a predetermined interval ofthe transient condition has been recorded on said medium.

4. In an arrangement for recording a transient circuit condition of anelectric circuit: an endlessly operating magnetic recording medium;

means for continuously supplying signal voltages 'of a frequencyreproducibly recordable on said magnetic recording medium andrepresentative of the conditions of said circuit; said recording mediumbeing cyclically operated at a predetermined rate correlated to thefrequency of said signal voltages; recording means for continuouslymagnetically recording on said recording medium a record of said signalvoltage and obliterating means for returning a portion of said medium toa uniform state of magnetization a relatively short time interval aftera record has been made thereon; and mean responsive to an occurrence ofa transient circuit condition on said circuit for discontinuing theaction of said obliterating means on said medium and operative a timeinterval thereafter for discontinuing the action of said recording meanson said medium.

5. A device for recording a transient which may occur at anunpredictable instant comprising, in combination, an endless recordingmedium, means for driving said endless medium, recording means connectedto the circuit in which the transient is expected for continuouslyrecording the circuit conditions, means for obliterating the recordedsignal, and means responsive to acircuit transient condition forrendering the obliterating means inoperative and for rendering therecording means inoperative after the transient condition has beenrecorded on the recording medium.

6. The method of visually presenting on an oscillagraph a non-repetitivewave which comprises the steps of: electrically recording thenonrepetitive wave on an endless medium, repeatedly reproducing saidrecord from said medium to establish a cyclic signal the frequency ofwhich corresponds to the rate of repetition of said record of saidnon-repetitive wave, and controlling the oscillograph by said cyclicsignal, the rate of occurrence of said record of said non-repetitivewave being sufliciently high that together with the persistence of theobservers eye the trace on the oscillograph corresponding to one cycleof said repetitive wave appears substantially steady.

7. The method of visually presenting on an oscillograph a wave of shortduration which comprises'the steps of: electrically recording the shortwave on a short endless medium, repeatedly electrically reproducing saidrecord from said short endless medium to establish a cyclic signal having a frequency which equals or exceeds the rate of repetition of saidrecord of said short Wave, and controlling the oscillograph by saidreproduced signal, the rate of repetition of said record of said shortwave being sufficiently high that together with the persistence of theobserver's eye the trace established by said oscillograph appearssubstantially steady.

8. A device for visually presenting on an oscillograph the wave form ofa non-repetitive electrical transient comprising; endless signal storagemeans, electrical recording means in association with said signalstorage means for recording thereon said non-repetitiv wave, means forrepeatedly rotating said endless signal storage means at a substantiallyconstant rate of speed, electrical reproducing means in association withsaid signal storage means for reproducing the record thereon upon eachrotation thereof to establish a cyclic signal the frequency of whichcorresponds to the rate of repetition of said record of saidnon-repetitive wave, means connecting said electrical reproducing meansto said oscillograph for controlling the oscillograph in accordance withsaid cyclic signal, the rate of occurrence of said record of saidnon-repetitive wave being sufiiciently high that together with thepersistence of the observers eye the trace on the oscillographcorresponding to one cycle of said non-repetitive wave appearssubstantially steady.

9. A device for visually presenting on an oscillograph the wave form ofan electrical transient of short duration comprising, endless signalstorage means, electrical recording means in association with saidsignal storage means for recording thereon said wave form of shortduration, means for repeatedly rotating said endless signal storagemeans at a substantially constant rate of speed, the period of rotationof said storage means being comparable to the duration of saidelectrical transient, electrical reproducing means in association withsaid signal storage means for reproducing the record thereon upon eachrotation thereof to establish a repeating wave, means connecting saidelectrical reproducing means to said oscillograph for controlling theoscillograph in accordance with said repeating wave, the rate ofoccurrence of said repeating wave being sufficiently high that the traceon the oscillograph appears substantially steady.

10. A device as set forth in claim 9 in which said transient of shortduration is less than one second long.

11. A device for use in recording a transient signal comprising, incombination, an endless magnetic record material, means for driving saidrecord material, magnetic recording means associated with said recordmaterial and connected to a circuit in which said transient is expected,said recording means being adapted to record on said record material asignal corresponding to the normal and abnormal conditions of thecircuit, magnetic obliterating means continuously obliterating threcorded signal during normal conditions of the circuit, means operableupon the occurrence of an abnormal condition for rendering saidobliterating means inoperative whereby a signal corresponding to theobnormal condition recorded on said record material is retained. andmeans operable upon the occurrence of the obnormal condition and after atime delay for rendering said recording means inoperative to prevent a.subsequent record from being superimposed on said recorded transientsignal.

12. A device for recording a transient signal comprising, incombination, endless record material, means" for cyclically driving saidrecord material, recording means associated with said record materialand connected to a circuit in which said transient is expected, saidrecording means being adapted to record on said record material a signalcorresponding to the conditions of the circuit, means for continuouslyerasing said record during normal conditions of the circuit, switchingmeans operable upon the occurrence of an abnormal condition forrendering said erasing means inoperative whereby a signal correspondingto the abnormal condition recorded on said record material is retained,and switching means operable upon the occurrence of the abnormalcondition and after a time delay of a duration no greater than thelength of time for one cycle of the record material for rendering saidrecording means inoperative to prevent a subsequent record from beingrecorded on said record means.

13. A device for recording a transient comprising in combination, anendless recording medium, means for cyclically driving said recordingmedium, recording means associated with said recording medium andadapted to be connected to a circuit in which a transient might occurfor recording the circuit conditions on said medium, obliterating meansfor destroying th recorded record after a short time interval, meansoperable upon the occurrence of a transient in the circuit for renderingsaid obliterating means inoperative, and means operable upon theoccurrence of said transient for rendering said recording meansinoperative after a short time delay 14. A device as set forth in claim13 in which said recording means includes a source of carrier currentand means for modulatin said carrier current by the transient.

15. A device for visually presenting on an oscilloscope the wave form ofa non-repetitive electrical transient comprising: endless magnetic tapemeans, magnetic means for recording the entire non-repetitive transienton said magnetic tape means, means for cyclically driving said magnetictape means, magnetic reproducing means actuated by the record of thenon-repetitive transient during the cyclic rotation of the endless tapemeans, said magnetic reproducing means controlling said oscilloscopemeans to repeatedly show the transient wave form, the rate ofreproduction of the wave form being sufiiciently high, and thepersistence of the screen and of the human eye being such that the traceon the oscilloscope screen appears substantially steady.

16. A device for converting a non-repetitive signal transient into asubstantially constant sig- 'nal which can be seen on an oscilloscopeunit comprising, an endless magnetic tape, means for magneticallyrecording the entire non-repetitive transient on said magnetic tape,magnetic reproducing means actuated by the record on the magnetic tapeand connected to the said oscilloscope unit for producing on theoscilloscope a visual trace of said entire non-repetitive transient, andmeans for repeatedly rotating said endles tape to cause said visualtrace to repeatedly appear on said oscilloscope, the rate of repetitionof said trace being sufliciently high, and the persistence of the screenand of the human eye being such that the trace on the screen of theoscilloscope unit appears substantially steady.

17. A device for use in recording a transient signal comprising incombination, an endless record member, means for driving said recordmember, recording means associated with said record member and connectedto a circuit in which said transient is expected, said recording meansincluding a source of carrier current and mean for modulating saidcarrier current by the signal in said circuit whereby said recordingmeans records continuously on said record member, obliterating meansassociated with said record member for continuously restoring saidrecord member to the condition it was in prior to recording, firstswitching means operable upon the event of a transient in said circuitfor rendering said obliterating means inoperative, and second switchingmeans operable upon th event of the transient in said circuit forrendering said recording means inoperative.

18. A device as set forth in claim 17 in which said record member iscyclically driven and the second switching means operates to render therecording means inoperative after a time delay which is equal to orslightly less than the time for one cycle of said record member wherebythe transient is retained on the record member,

19. In a system for reproducing a characteristic image of an electrictransient signal: an endless recording medium; means for providing acarrier current of a frequency reproducibly recordable on said recordingmedium; said recording medium being cyclically operated at apredetermined rate correlated to the frequency of the carrier current;modulator means for causing said signal to modulate said carrier currentand providing a modulated signal; said modulator means being arranged tosubstantially balance out the said transient signal; recording means forrecording said modulated signal on said recording medium; said recordingmeans including means for amplifying said modulated signal and filtermeans interposed in the path of the amplified modulated signal anddesigned to pass a range of frequencies including the frequency of themodulated carrier current and side bands corresponding to said transientsignal and to restrict the passage of other frequencies so that thesignals of the passed frequency range are efiectively recorded;reproducing means including amplifying means for reproducing saidmodulated signal from said recording medium and for amplifying thereproduced signal; said reproducing means including filter means de-'signed to pass a range of frequencies including the frequency of themodulated carrier current and side bands corresponding to said transientsignal and to restrict other frequencies and demodulator means fordemodulating the reproduced signals passed by said filter means and socorrelated to the other elements of the system that the demodulatedreproduced signals correspond in a predetermined manner to said originaltransient; and visual indicating means actuated by said demodulatedreproduced signals; the rate of the cyclical operation of the recordingmedium being correlated to the characteristics of the indicating meansso that in response to actuation by said demodulated reproduced signalsit produces a visually observable characteristic image of said transientsignal.

20. In a system for reproducing a characteristic image of a transientsignal: an endless recording medium; means for providing a carriercurrent; said recording medium being cyclically operated at apredetermined rate correlated to the frequency of the carrier current;modulator means adapted to combine said carrier current and saidtransient signal and cause said signal to modulate said carrier currentand provide a modulated signal; said modulator means being arranged tosubstantially balance out the said transient signal; recording meansincluding means for amplifying said modulated signal and means forrecording said modulated signal on said recording medium; reproducingmean including means for reproducing said modulated signal from saidrecording means and means for amplifying said reproduced signal; saidreproducing means including means for demodulating said modulated signalso as to provide an image signal corresponding to said originaltransient signal; and visual indicating means actuated by said imagesignal; the rate of the cyclical operation of the recording medium beingcorrelated to the characteristics of the indicating means so that inresponse to actuation by said demodulated reproduced signals it producesa visually observable characteristic image of said transient signal.

21. In the method of recording a transient circuit condition which mayoccur at an unpredictable instant on an electric circuit subjected topredetermined normal circuit conditions, the procedure of continuouslysupplying signal voltages of a frequency reproducibly recordable on amagnetic recording-medium and representative of the conditions of saidcircuit; continuously magnetically recording on a cyclically operatingendless magnetic recording medium a record of said signal voltages andcontinuously obliterating said record; stopping further obliteration ofsaid record upon the occurrence of a transient circuit condition; andstopping further recording a predetermined time after the obliterationof the record was stopped.

22. In the method of observing a transient signal, the procedure of:providing a carrier current of a frequency reproducibly-recordable on amagnetic recording medium; producing a modulated signal by modulatingsaid carrier current by the transient signal; recording the modulatedsignal on a magnetic recording medium which is cyclically operated at afrequenc correlated to the carrier frequency; cyclically reproducingfrom the recording medium the recorded modulated sign-a1 anddemodulating the reproduced signal; and controlling an indicating devicein accordance with said cyclicall reproduced demodulated signal.

23. In the method of observing a transient signal, the procedure ofproviding a carrier current of a frequency reproducibly-recordable on amagnetic recording medium; producing a modulated signal by modulatingsaid carrier current by the transient signal; recording the modulatedsignal on a magnetic recording medium which is cyclically operated at afrequency correlated to the carrier frequency; cyclicall reproducingfrom the recording medium the recorded modulated signal and demodulatingthe reproduced signal; and controlling a trace-producing electron beamby the cyclically reproduced demodulated signal so as to provide avisually observable trace.

24. In the method of observing a signal, the procedure of: providing acarrier current. of a frequency reproducibly-recordable on a magneticrecording medium; producing a modulated signal by modulating saidcarrier current by the signal; recording the modulated signal on amagnetic recording medium which is cyclically operated alt-"a, frequencycorrelated to the carrier fre- .providea visually observable trace.

25. In a system for studying a transient signal; an endless magneticrecording medium; a source of carrier current of a frequencyreproduciblyrecordable on said recording medium; said mag- I neticrecording medium being cyclically operated at a predetermined ratecorrelated to the frequency of said carrier current; modulator means forcausing said signal to modulate said carrier current by said transientsignal and providing a correspondingly modulated signal; recording meansfor magneticall recording on said medium a signal corresponding to saidmodulated signal; reproducing means including demodulating means forcyclically reproducing from said medium the recorded modulated signaland for demodulating the reproduced signal so as to cyclically provide ademodulated reproduced signal corresponding to the transient signal; andcyclically-operative indicating means cyclically actuated by saiddemodulated reproduced signal for presenting said reproduced signal uponeach cycle of said medium.

26. In a system for studying a transient signal: an endlesscyclically-operating magnetic recording medium; a source of carriercurrent of a frequency reproducibly-recordable on said medium; modulatormeans adapted to modulate said carrier current by said transient signalfor providing a correspondingly modulated carrier current; magneticrecording means arranged to continuously record on said medium amagnetic si nal corresponding to the said carrier current and operativeupon the occurrence of a transient to record on said medium a modulatedsignal corresponding to the said modulated carrier current; magneticobliterating means spaced from said recording means for continuouslyobliterating the recorded signal so that said endless medium alwaysincludes a short medium portion corresponding to the space between therecording and obliterating means upon which the record has beenobliterated and a medium portion upon which a record has been made;means actuated upon the occurence of a transient to first render saidobliterating means inoperative and to render, a relatively short timeinterval thereafter, said reproducing means inoperative so as to retainon said medium a record of a modulated signal corresponding to thetransient signal; oscilloscope means; and reproducing means includingdemodulating means associated with said medium and interconected withsaid oscilloscope means for cyclically reproducing the signals recordedon said medium during each entire cycle and for cyclically impressing onsaid oscilloscope means unmodulated or modulated si nals correspondingto the signals reproduced from said medium so as to indicateovermodulation of the carrier by comparing the recorded and unrecordedportions of the medium.

27. In the method of recording a transient cincuit condition of anelectric circuit, the procedure of: continuously supplying signalvoltages of a frequency reproducibly-recordable on a magnetic recordingmedium and representative of the conditions of said circuit;continuously magnetically recordin on a cyclically-operating endlessmagnetic medium a record of said signal voltages; obliterating eachrecord element recorded on the medium prior to the end of the cycleduring which it was recorded; stopping obliteration of said record uponthe occurrence of a transient circuit condition which it is desired torecord; and stoppin further recording a predetermined time afterstopping the obliteration.

28. In a system for studying a transient signal: an endless magneticrecording medium; a source of carrier current of a frequencyreproduciblyrecordable on said recording medium; said magnetic recordingmedium being cyclically operated at a predetermined rate correlated tothe frequency of said carrier current; modulator means for causing saidsignal to modulate said carrier current by said transient signal andproviding a correspondingly modulated signal; recording means formagnetically recording on said medium a signal corresponding to saidmodulated signal; reproducing means including demodulating means forcyclically reproducing from said medium the recorded modulated signaland for demodulating the reproduced signal so as to cyclically provide ademodulated reproduced signal corresponding to the transient signal; andoscilloscope means cyclically actuated by said reproduced demodulatedsignal for presenting said signal upon'each cycle of said medium, thecyclical rate of said medium being sufficiently high so that asubstantially steady picture of said signal is produced on saidoscilloscope.

' 29. A system as defined in claim 28 in which said transient signal isa direct current signal.

30. A system as defined in claim 28 in which said carrier current has afrequency sufficiently low that when modulated by an alternatingtransient signal the resulting upper side band is within the range whichcan be magnetically recorded and reproduced. 1

31. A system as defined in claim 28 in which said carrier current has afrequency sufliciently high that when modulated by a very low or directcurrent transient signal the resulting lower side band is within therange which can be magnetically recorded and reproduced.

SEMI JOSEPH BEGUN.

